Dehydration behaviour of the Ca(SO4,HPO4).2H2O solid solution

Abstract

Gypsum (CaSO4·2H2O) and brushite (CaHPO4·2H2O) are minerals with similar structures, despite not being isostructural. They both belong to the monoclinic system, but crystallize in different space groups. It is known that, to some extent, phosphate and sulphate groups can substitute for each other within the structure of these minerals. In the present work, the dehydration behaviour of experimentally produced Ca(SO4,HPO4)·2H2O solid solutions is assessed from thermogravimetrical (TG and DTG) and differential scanning calorimetry (DSC) analyses. For the applied experimental conditions it was not possible to obtain homogenous precipitates with compositions within 0.27 < XBr < 0.53, where XBr is the brushite mole fraction. The results reveal that the dehydration behaviour of solid solutions with compositions within the 0 < XBr < 0.27 interval tend to approach the dehydration process of pure gypsum. On the other hand, solid solutions with compositions within 0.53 < XBr < 1 involve higher dehydration temperatures than the brushite end-member, in a two-step process.